Adult intrahepatic progenitor/stem cells capable of producing both bile ducts and hepatocytes during injury responses are of great importance in liver biology, both in health and disease. During the past funding period we have finally been able to home in on this long elusive cell population. It is now possible to prospectively isolate te intrahepatic stem cell of mice by FACS, to culture it in vitro and to perform lineage tracing in vivo. We have performed gene expression profiling on LPCs both in the resting state and after activation of the oval cell response. This analysis has revealed several novel genes unique to this population, suggesting that they play important roles in their maintenance and/or function. Using genetic tools it is therefore now possible to explore the function of these factors in liver progenitors. This information will be useful to control and manipulate the behavior of LPC both in vivo and in vitro for the purposes of regenerative medicine. We also have been able to establish expandable cultures of human LPC and thus will be able to translate lessons from the murine system to human biology. In this application, we propose to systematically explore the function of key genes unique to adult LPC (Aim1), to use genomic approaches to understand the oval cell activation process in greater depth (Aim2) and to test the utility of human LPC as an expandable source of hepatocytes (Aim3) . Successful completion of this research will result deepen our understanding of the molecular controls governing adult liver progenitors and make them amenable to therapeutic manipulations.